1. Field of the Invention
The present invention relates to a driving circuit for driving a driven element, a driving apparatus including the driving circuit and the driven element, and an image forming apparatus including the driving apparatus.
2. Description of the Related Art
Driven elements such as light-emitting diodes (LEDs), organic electroluminescence elements (organic light-emitting diodes or OLEDs), and light-emitting thyristors generally have temperature dependencies. The LEDs used as light-emitting elements in the optical print heads (LED heads) of LED printers, which are a type of electrophotographic printer, emit less optical power as their temperature rises. Since the printing darkness of an LED printer varies depending on the emitted optical power, the LED drive current must be changed to compensate for variations in emitted optical power caused by such temperature changes.
As disclosed, for example, by Nagumo in U.S. Pat. No. 6,028,472 (Japanese Patent Application Publication No. H10-332494) and Japanese Patent Application Publication No. 2006-159472 (now Japanese patent No. 4498905), an LED printer has a plurality of LEDs, driver integrated circuits (ICs) for feeding drive current to the LEDs, and a reference voltage generating circuit for supplying a reference voltage to the driver ICs. The drive current fed to the LEDs is proportional to the reference voltage applied to the driver ICs, so for temperature compensation, the reference voltage generating circuit in the above disclosures operate with a positive temperature coefficient that causes the reference voltage to increase as the temperature rises.
The reference voltage generating circuit disclosed in U.S. Pat. No. 6,028,472 outputs a voltage substantially proportional to absolute temperature. The reference voltage generating circuit disclosed in JP 2006-159472 provides a temperature coefficient that can be set to different values by selection of suitable components.
An LED head must hold the emitted optical power of the LEDs at the prescribed level as the LED temperature rises even if the temperature rise is due to the driving of the LEDs. There is also a need for a temperature compensation circuit having a temperature coefficient that is flexibly settable according to the temperature characteristics and luminous efficiency of the LEDs, which vary depending on the crystalline material and emission wavelength.
For LEDs with some characteristics, the reference voltage generating circuit in U.S. Pat. No. 6,028,472, which outputs a reference voltage proportional to absolute temperature, is unable to perform appropriate temperature compensation.
JP 2006-159472 discloses a reference voltage generating circuit with an internal diode-generated forward voltage drop. The voltage drop has a temperature coefficient that compensates for the temperature coefficient of the LEDs, but if the voltage drop is made large enough to obtain an adequate range of compensation, the reference voltage supplied to the driver ICs is reduced to such a low value that voltage noise effects etc. become significant. In the presence of such noise effects, it becomes difficult to specify a reference voltage that produces the desired output from the LEDs.